The present invention relates to novel derivatives of 1,2,3,4-tetrahydro-9-acridinamine useful as pharmaceutical agents, to methods for their production, to pharmaceutical compositions which include these compounds and a pharmaceutically acceptable carrier, and to pharmaceutical methods of treatment. More particularly, the novel compounds of the present invention are derivatives of 1,2,3,4-tetra -hydro-9-acridinamine which are useful in treating the symptoms of cognitive decline in an elderly patient.
Disorders of cognition are generally characterized by symptoms of forgetfulness, confusion, memory loss, attentional deficits and/or, in some cases, affective disturbances. These symptoms may arise as a result of the general aging process and/or from organic brain disease, cerebrovascular disease, head injury or developmental or genetic defects.
The general decrease in cognitive function which accompanies the aging process is well accepted. The same phenomenon has been observed and documented in many lower mammals, including those routinely employed in pharmacological testing programs for screening and predicting usefulness for particular drugs in higher animals, including humans.
Although disorders of cognition often accompany the general aging process, presenile and senile primary degenerative dementia are the most common accepted causes of mental deterioration in the elderly. It has been estimated that at least ten percent of persons over sixty years of age will eventually suffer severe mental deterioration. A much larger number will experience cognitive decline of sufficient severity to impede their activities.
Many of the symptoms of cognitive disorders, especially impaired memory, are associated with decreased acetylcholine synthesis and the impairment of cholinoreceptive neurons. In the hippocampus and cerebral cortex of patients suffering from primary degenerative dementia for example, the level of the enzyme choline acetyltransferase (CAT) can be reduced as much as ninety percent (see Davies, P., et al, The Lancet, 2, page 1403 (1976); Perry, E. K., et al, Journal of Neurological Sciences, 34, pages 247 to 265 (1977): and White, P., et al, The Lancet, 1, pages 668 to 670 (1977)).
Since CAT catalyzes the synthesis of acetylcholine from its precursors choline and acetyl coenzyme A, the loss of CAT reflects the loss of cholinergic or acetylcholine-releasing nerve endings in the hippocampus and cerebral cortex. There is abundant evidence that cholinergic terminals in the hippocampus are critically important for memory formation.
The cholinergic hypothesis suggests that drugs which restore acetylcholine levels or cholinergic function (i.e., cholinomimetic) are effective in correcting this deficit in neurotransmitter chemical and provide treatment of the memory impairment symptom of cerebral insufficiency. Considerable biochemical, pharmacological, and electrophysiological evidence supports the hypothesis that deficits in the cholinergic system underlie geriatric cognitive dysfunction (Peterson, C. and Gibson, G. E., Neurobiology of Aging, 4, pages 25 to 30 (1983)). Aged humans and nonhuman primates with decreased cognition show improved memory when they are treated, for example, with acetylcholinesterase inhibitors such as 1,2,3,4-tetrahydro-9-acridinamine and physostigmine. These agents increase the available supply of synaptic acetylcholine by inhibiting its hydrolysis.
1,2,3,4-Tetrahydro-9-acridinamine (tacrine; THA) has been shown to be useful in the long-term palliative treatment of patients with Alzheimer's disease (Summers, W. K., et al, The New England Journal of Medicine, 315, pages 1241 to 1245 (1986)). The results indicated that when the dose of tacrine was increased up to 200 mg per day the clinical response improved dramatically. However, toxic side effects also were noted when large doses of tacrine were used. Thus, there is a need to administer a therapeutically effective amount of tacrine over a prolonged period of time to a patient without the concomitant undesirable toxic side effects.
Ideally, if a drug is delivered to the target area in a controlled amount, therapy should be maximized and toxic side effects minimized.
One method of controlling the release of a drug is to derivatize as a prodrug or to localize the drug in some biological depot or site within the organism with subsequent slow release of the drug in a therapeutically effective quantity over an extended period of time. Various biologically active agents have been modified chemically to form prodrugs or depot derivatives. The derivative is then administered to a patient, localized in a biological depot and subsequently biotransformed in vivo into the active agent over an extended period of time.
A prodrug or a depot derivative has several advantages, one of which is that the patient is exposed to less total active drug in any given period of time which minimizes or eliminates local or systemic side effects. Additionally, there is a decrease in the frequency with which the patient has to take the drug. This is particularly important in the case of a patient suffering from Alzheimer's disease where patient compliance is a problem. However, the chemically altered prodrug or depot derivative may result in a drug with a different pharmacological profile than that found in the parent drug. We have found unexpectedly that certain 9-substituted amino derivatives of tacrine act as prodrugs or depot agents and release tacrine in vivo over a prolonged period of time and are thus useful in the long-term treatment of patients with Alzheimer's disease.